Make judicious use of U+1F42B on darwin.

Only tested on osx 10.8, maybe 10.7 has the glyph. Before is hopless,
at best unassigned code point glyph at worse garbage. Also it assumes
an UTF-8 terminal. Well... if rejected, just an opportunity to plug the code point
beyond uucd's docs.


![opam-bactrian-log](https://f.cloud.github.com/assets/485596/77900/1aa48722-6161-11e2-8ebd-db4e4aa293a0.png)